The art of making integral low density cellular polymeric structures having uniform small voids from expandable polymeric materials is well established. Thermoplastic foam products are generally formed by processing tiny beads or spherical particles of a thermoplastic resin impregnated with an expanding agent. The particles expand when exposed to heat, thereby forming a foam structure.
Various means have been suggested for heating the beads to cause them to expand into a foamed structure. Hot water has been used, but such treatment is limited in its commercial application to situations wherein the material is foamed in molds, or other applications where it is not prepared in situ with materials of various types which would not stand exposure to water. The hot water technique is also limited with respect to time, sometimes requiring several hours to produce the foamed structure.
Infrared rays have also been used for expansion of thermoplastic resin beads. The tendency in this method is to overheat one side of the bead, thereby causing collapse of the foamed structure and increased density, the heated side having a high density and the unheated side having a low density. Difficulties are also encountered when an effort is made to form the foamed structure in situ using infrared heat.
Steam has also been extensively used to promote the expansion of beads. However, the use of steam is limited by the maximum temperature achievable by the steam available. Because of these limitations steam is not an efficient heat source for processing transition temperature. Additionally, the use of steam presents the difficulties of entrained steam in the products and the reaction of steam with polymers.
It has been proposed that other methods of heating the polymeric beads be employed.
Edberg, et al., U.S. Pat. No. 3,242,238, disclose treating or covering polymer beads with an aqueous solution of a wetting agent and then exposing the beads to dielectric heating, i.e. a high frequency heating unit operating at about 70 megacycles per second. However no teaching of employing very high frequency energy to heat such beads or of the novel very high frequency energy absorbing organic materials is disclosed.
Nazar, et al., U.S. Pat. No. 4,765,934 disclose uniformly distributing a water soluble salt over the surface of polystyrene beads and applying microwave energy to effect foaming. The patentees, however, do not teach employing very high frequency energy absorbing organic materials.
It has now surprisingly been discovered that expandable thermoplastic resin beads coated with a very high frequency energy absorbing organic material or its water solution or organic material coated on a substrate produce excellent foamed articles. Application of very high frequency energy to the coated beads of the present invention provides rapid heating and expansion and is amenable to beads comprising engineering plastics having a high glass transition temperature.
Also to be mentioned are Dench, U.S. Pat. No. 3,848,038 describing drying thermoplastic resins with microwave energy and Thorsrud et al., U.S. Pat. No. 3,234,636 describing employing microwave energy in a flow molding process.